OF SECRETION. 155 



the inside of the trachea is in constant action, and that that action is 

 always directing the substance attached to it in its own direction. 

 This might be attempted to be illustrated by the effect produced on a 

 hair when rubbed between the fingers and thumb ; for, although the 

 motion in the finger and thumb does not direct the hair to take any 

 course, yet it moves in one direction. This arises from the surface of 

 the hah'. But the same thing would happen if the fingers had the 

 same kind of surface with the hair, and the hair the same with the 

 fingers. It might be supposed that the air passing out might give this 

 direction [to the mucus in the trachea] ; but I should hardly conceive 

 that sufficient ; for the air must also pass the contrary way, which, if 

 equal in current, would keep it stationary. However, we may observe 

 that the air passes out with greater velocity than it passes in. 



Organs of Secretion. 



Secretion cannot be called fermentation, nor can it be called filtra- 

 tion, but it is a separation of such parts of the blood as the particular 

 glands are fitted for ; and such as are either obnoxious to the constitu- 

 tion, or, as when combined, or united again, have properties according 

 to the parts separated. 



In all our secretions there is a great deal of that substance called 

 mucus : indeed mucus would seem to be the basis of most. The other 

 ingredients are the distinguishing marks by which means they are 

 called ' this' or ' that.' The urine, sweat, perspiration, and the tears, 

 would seem to contradict this opinion; but there is mucus in all of 

 them. The formation of mucus seems to be the natural animal change 

 into which she forms herself when she intends throwing part of herself 

 out of the constitution, either for waste or for other purposes, such as 

 digestion and motion of the parts already formed : so that most of the 

 secretions that are to answer some particular purpose, as the bile, 

 semen, &c, have the particular property mixed with mucus. 



In many of our secretions, there is mixed with the mucus something 

 which gives us the idea of bitter. The bile has this constantly, but 

 less in some animals than others ; and less at one time than another. 

 The wax of the ear has it. The mucus on the tongue in a fever, or 

 after a debauch, has it. I should be apt to suspect that this principle 

 is of a vegetable nature, similar to the bitters of that kingdom. 



Of the Liver [loose note] . 



From the liver being but one long lobe in snakes 1 , &c, it would 

 appear that the vast division in the liver of the dog 2 , &c., is not for 



1 [Hunt. Prep. No. 802.] 2 [ib. No. 806 (Cat).] 



